1. Field of the Invention
The present invention relates to an optical recording medium, a method of producing the same and a method of producing an optical recording card, and more particularly to a writable type (DRAW type) optical recording medium, a method of producing the same and a method of producing an optical recording card.
The optical recording medium is used when sound, image and other type of signal are recorded optically, and it is utilized in the form of a disc or card. In the recent years, a card in which various kinds of informations are recorded are widely put in practical use as ID card, cash card or bank card.
2. Prior Art
A disc having an optical recording medium incorporated therein is widely used as a recording member in which digital signals relative to sound and image are recorded. This kind of card is required to record various kinds of informations such as individual data, data concerning an issuance company or the like. In the eariler age, such kinds of informations were recorded using visual characters and symbols and in the later age they are recorded using electrical signals which are produced with the aid of magnetism. However, in a case where magnetism recording medium is used, recorded informations are easy to be falsified. Accordingly, there is a need of taking adequate actions for preventing recorded informations from being falsified and moreover dealing with the current increased quantity of informations to be recorded.
To this end, an optical recording card to which laser technique is applied has been lately developed. This optical recording card is such that it is provided with an information recording medium which has an optical reflection surface. Lately, a writable type (DRAW type) information recording medium has been required in addition to a conventional read only memory type (ROM type) recording medium.
As shown in FIG. 7, a conventional DRAW type optical recording card 101 is so constructed that a gelatine layer 103 with silver gains serving as optical recording medium is formed on a transparent card front board 102, a transparent film 105 having printed patterns 104 formed thereon is adhesively secured to the bottom surface of the gelatine layer 103 with silver grains with the use of a transparent adhesive 106 and a card rear board 107 is adhesively secured to the bottom surface of the transparent film 105 as viewed in the drawing with the use of a transparent adhesive 108.
A laser beam for the purpose of writing is introduced from a card front board 102 side to melt the gelatine layer 103 with silver grains, causing the non-reflective layer to be exposed to the outside, whereby a pit is formed in which informations are recorded. Reading of the recorded informations is achieved by discriminating "0" from "1" in dependence on an intensity ratio of reflected lights from the pit and the gelatine layer 103 with silver grains.
A structure of the DRAW type optical recording medium and a method of producing the same are disclosed, for instance, in an official gazette of Japanase Patent Publication NO. 23716/1984. However, in the conventional optical recording medium, silver halide emulsion is used for forming the gelatine layer 103 with silver grains and formation of the latter requires strict control relative to developing conditions and many complicated steps of exposing and developing, resulting in it being produced at an expensive cost. Further, the conventional optical recording medium is such that the recording portion is circularly recessed under the effect of energy of laser beam and the non-reflective layer is exposed to the outside to build a recording part. In the conventional optical recording medium, an intensity of reading light is weak in the range of 1/5 to 1/10 of that of recording light but the non-recording portion is brought in a state close to the recording portion under the influence of accumulative energy with the result that a ratio of S/N is reduced and preservability of recorded informations is degraded. As another optical recording medium, there are known those that utilize metallic brightness of cyanine dyestuff film and absorbability of laser beam and those that are so constructed that PVA or gelatine layer serving as a photosensibility increasing layer is superposed on the cyanin dyestuff film in a contacted state. In these cases, in order to effect preformatting, patterning of the dyestuff film or patterning of the reflection amplifying film is required, resulting in a process of production becoming complicated. Also in these cases, the above-mentioned materials are used as recording material by utilizing their thermal deformation, fading, decomposition or the like but reduction of S/N is brought about as weak reading light is repeatedly radiated by many times. Consequently, reservability of recorded informations is degraded.
Accordingly, it is preferable that an optical recording medium is not material which continuously varies relative to beam power but it is material with which presence and absence of recorded informations is recognized in the presence of a certain "threshold".